1. Field of the Invention
The present invention relates to a liquid crystal display device mounting a direct light source on a liquid crystal display element, and more particularly, to a liquid crystal display device which can irradiate the illumination light of the uniform brightness with respect to the liquid crystal display element by suppressing a warp of a diffusion plate which is arranged between the direct light source and the liquid crystal display element.
2. Description of the Related Art
There has been known a liquid crystal display device which is provided with a light source which illuminates a liquid crystal panel for observing an electronic tent image formed on a liquid crystal panel as a clear visible image. This type of light source is a so-called backlight which constitutes a light source which illuminates the liquid crystal display element from a back surface thereof. Hereinafter, the light source is also referred to as “backlight”.
As such a backlight, a side-edge backlight which arranges a linear lamp (a fluorescent lamp, particularly a cold cathode fluorescent lamp being used popularly) at a side surface of a light guide plate made of a transparent plate formed by molding using acrylic resin or the like, and a direct backlight which arranges one or a plurality of linear lamps right below a back surface of the liquid crystal panel have been known.
A notebook type computer which is required to be as thin as possible adopts the side-edge backlight and a liquid crystal display monitor for a liquid crystal display device also mainly adopts the side-edge backlight to shorten a depth of the display device.
With respect to a large-sized liquid crystal display device such as a display monitor, the acquisition of a high contrast and bright color display image and the prevention of the deterioration of brightness even after the use for a long time constitute inevitable requirements and hence, a liquid crystal display device of a type (direct type) which arranges a plurality of linear lamps right below the liquid crystal panel has been commercialized.
FIG. 14 is a cross-sectional view which is served for explaining a constitutional example of the liquid crystal display device which is provided with the direct backlight. In the drawing, PNL indicates a liquid crystal display element (a liquid crystal panel) which generates an image electronically. The liquid crystal display element sandwiches a liquid crystal layer LC between a pair of glass substrates SUB1, SUB2 and generates an image by selectively applying voltage to electrodes or switching elements for pixel selecting which are formed on either one or both of the glass substrates SUB1, SUB2.
Further, the polarizers PL1, PL2 are laminated to respective outer surfaces of the glass substrates SUB1, SUB2. The polarizers PL1, PL2 control the polarization of the illumination light from a backlight BL so as to make the light which passes through the liquid crystal layer LC irradiate from the upper-side polarizer (PL2) or interrupt the irradiation.
The backlight BL is constituted of a plurality of cold cathode fluorescent lamps CFL, a reflection plate REF, a diffusion plate SCT which controls the distribution of illumination light irradiated from the cold cathode fluorescent lamps CFL, and an optical sheet OPS which is constituted of a laminated body formed of at least one diffusion sheet SC which controls the direction of the illumination light and at least one prism sheet PRS. The backlight BL is mounted on a back surface of the liquid crystal display device PNL.
FIG. 15 is a cross-sectional view which is served for schematically explaining a specific example of the backlight BL shown in FIG. 14. In the direct backlight, the diffusion plate SCT which has a relatively large thickness and is formed of an acrylic resin plate, a polycarbonate resin plate or the like is arranged above and close to a plurality of cold cathode fluorescent lamps CFL which constitute the light source.
Further, on a surface of the diffusion plate SCT which is disposed right above and faces the cold cathode fluorescent lamps CFL in an opposed manner, a reflection light shielding pattern for correcting the brightness irregularities is formed by printing or the like. The reflection light shielding pattern is adjusted to make the illumination light which illuminates the liquid crystal display element obtain the optimum brightness distribution when the diffusion plate is flat.
The cold cathode fluorescent lamps CFL which constitute the backlight are mounted along valley portions of mountain-like reflection plates REF arranged in the inside of a metal-made lower frame FLM-D made of an aluminum plate or an iron plate. On the diffusion plate SCT which is arranged above the cold cathode fluorescent lamps CFL, the optical sheet OPS which is constituted of a laminated body consisting of at least one diffusion sheet SC and at least one prism sheet PRS is mounted in a superposed manner. Then, an upper frame FLM-U is engaged with and integrally formed with a lower frame FLM-D.
However, since the diffusion plate SCT is arranged close to the cold cathode fluorescent lamps CFL, when a given time elapses after the cold cathode fluorescent lamps CFL are turned on, a warp which forms the diffusion plate SCT in a projected form toward the liquid crystal display element side, for example, is generated so that there is a possibility that the flatness of the diffusion plate SCT is spoiled. When the flatness of the diffusion plate SCT is spoiled, the brightness distribution adjustment using the reflection light shielding pattern formed on the diffusion plate SCT gives rise to irregularities so that the illumination light irradiated to the liquid crystal display element cannot obtain the uniform brightness distribution.
To cope with such an inconvenience, a liquid crystal display device which arranges a light transmission sheet (hereinafter called “a transmission sheet”) at a back surface (a light source side) of the diffusion plate has been proposed (Japanese Laid-open Patent Publication 223812/1999). In this related art, a transparent sheet is brought into close contact with a back surface of the diffusion plate so as to decrease the contact of a light source side of the diffusion plate with outside air whereby the drying of the light source side of the diffusion plate is suppressed thus preventing the deformation of the diffusion plate.
Further, with respect to the deformation of the diffusion plate, Japanese Laid-open Patent Publication 326517/1998 discloses a liquid crystal display device which installs support columns (spacers) between a frame and a diffusion plate of a light source to prevent the downward deflection of a center portion due to the deadweight of the diffusion plate generated along with the lapse of time due to the use of the liquid crystal display device for a long period.
As other literatures which disclose the related art on the direct liquid crystal display device, Japanese Accepted Patent Publication 13666/1976, Japanese Laid-open Patent Publication 309921/1988 and the like can be named, for example.